The Yosemite Decimal System (YDS) of grading routes was developed at Tahquitz by members of the RCS of the Angeles Chapter of the Sierra Club in the 1950s. Royal Robbins in 1952 established what was then one of the hardest free climbs in the United States, known as "Open Book". As new routes were constantly being discovered and climbed, the old method of rating climbs as either easy, moderate, or hard was quickly becoming useless. With the help of fellow RCS members Don Wilson and Chuck Wilts, the modern system of rating fifth-class climbing was developed, with the scale running from 5.0 to 5.9. Mark Powell, a local who frequented Yosemite, passed this system on to the climbers in Yosemite. By the early 1960s, the Yosemite Decimal System was the standard in the United States.

The above was taken from Wikipedia and shows why Ewbank mentioned the Tahquitz grading system and its limitations.